Rapid synthesis of a catalyst comprising a zeolite having an afx structure and at least one transition metal for selective NOX reduction

Abstract

The invention relates to a process for preparing a catalyst that is based on a zeolite having an AFX structure and at least one transition metal, said process comprising at least the following steps: i) in an aqueous medium, mixing at least one source of silicon in oxide form SiO2, at least one source of aluminum in oxide form Al2O3, an organic nitrogen-containing compound R, and at least one source of at least one alkali and / or alkaline earth metal M, until a homogeneous precursor gel is obtained; ii) hydrothermally treating said precursor gel in order to obtain a crystallized solid phase; iii) at least one ion exchange with a transition metal; and iv) thermal treatment. The invention also relates to the catalyst that can be obtained or is obtained directly by said process and to a process for selective NOx reduction using said catalyst.

Description

TECHNICAL FIELD

[0001] The invention relates to a process for preparing a catalyst based on an AFX structural type zeolite obtained by rapid synthesis and at least one transition metal. PREVIOUS TECHNIQUE

[0002] Nitrogen oxide (NOx) emissions from the combustion of fossil fuels are a major concern for society. Increasingly stringent standards are being implemented by government bodies to limit the impact of combustion emissions on the environment and human health. For light vehicles in Europe, under the Euro 6c / Euro 6d-temp regulations, NOx and particulate emissions must reach very low levels under all operating conditions. The new WLTC (Worldwide Harmonized Light Vehicles Test Cycle) and the Real Driving Emissions (RDE) regulations, along with their associated conformity factors, require the development of highly efficient emissions control systems to achieve these objectives.Selective catalytic reduction, known by the acronym "SCR," is an effective technology for removing nitrogen oxides from the oxygen-rich exhaust gases typical of diesel and spark-ignition engines running on lean mixtures. Selective catalytic reduction is achieved using a reducing agent, usually ammonia, and can thus be referred to as NH3-SCR. The ammonia (NH3) involved in the SCR process is generally generated through the decomposition of an aqueous urea solution (AdBlue or DEF), and produces N2 and H2O upon reaction with NOx.

[0003] Transition metal-exchanged zeolites are notably used as catalysts for NH3-SCR applications in transportation. Small-pore zeolites, particularly copper-exchanged chabazites, are especially well-suited. They are commercially available as silico-aluminophosphate Cu-SAPO-34 and aluminosilicates Cu-SSZ-13 (or Cu-SSZ-62). Their hydrothermal stability and NOx conversion efficiency make them the current benchmarks. However, with increasingly stringent standards, catalyst performance still needs improvement.

[0004] The use of AFX structural type zeolites for NH3-SCR applications is known, but few studies evaluate the efficiency of catalysts using this zeolite.

[0005] Fickel et al. (Fickel, DW, & Lobo, RF (2009), The Journal of Physical Chemistry C, 114(3), 1633-1640) investigated the use of a copper-exchanged SSZ-16 (AFX structural type) for NOx removal. This zeolite is synthesized according to US patent 5,194,235, in which copper is introduced by ion exchange using copper(II) sulfate at 80°C for 1 h. Recent results (Fickel, DW, D'Addio, E., Lauterbach, JA, & Lobo, RF (2011), 102(3), 441-448) show excellent conversion and good hydrothermal stability for a 3.78 wt% copper loading.

[0006] Work on the synthesis of AFX structural type zeolites has been carried out with different organic structural agents (Lobo, RF, Zones, SI, & Medrud, RC (1996), Chemistry of materials, 8(10), 2409-2411, Martin Nuria et al., APPLIED CATALYSIS B: ENVIRONMENTAL, vol. 217, 29 May 2017, pages 125-136) as well as work on optimizing the synthesis (Hrabanek, P., Zikanova, A., Supinkova, T., Drahokoupil, J., Fila, V., Lhotka, M., Bernauer, B. (2016), Microporous and Mesoporous Materials, 228, 107-115).

[0007] Wang et al. (Wang, D. et al., CrystEngComm., (2016), 18(6), 1000-1008) investigated the replacement of the TMHD structuring agent with a TEA-TMA mixture for the synthesis of silicoaluminophosphate SAPO-56 and obtained undesired phases SAPO-34 and SAPO-20. The incorporation of transition metals is not addressed.

[0008] US patent application 2016 / 0137518 describes a near-pure AFX zeolite, its synthesis from silica and alumina sources in the presence of a 1,3-Bis(1-adamantyl)imidazolium hydroxide-type structuring agent, the preparation of a catalyst based on AFX zeolite exchanged with a transition metal, and its use for NH3-SCR applications. No specific form of AFX zeolite is mentioned.

[0009] More recently, US patent application 2018 / 0093259 describes the synthesis of small-pore zeolites, such as AFX structural zeolite, from FAU-type zeolite in the presence of an organic structuring agent, such as 1,3-bis(1-adamantyl)imidazolium hydroxide, and an alkaline earth metal source. It also describes applications of the resulting AFX structural zeolite, particularly its use as a NOx reduction catalyst after exchange with a metal such as iron. In parallel, US application 2016 / 0096169A1 presents the use in the conversion of NOx, of a catalyst based on a structural type AFX zeolite having a Si / Al ratio ranging from 15 to 50 exchanged with a metal, the AFX zeolite being obtained from a structuring agent of the hydroxide type of 1,3-Bis(1-adamantyl)imidazolium.The results obtained in the conversion of NOx show in particular a selectivity of the catalysts prepared according to US applications 2018 / 0093259 and US 2016 / 0096169 towards nitrous oxide not exceeding 20 ppm.

[0010] Document JP 2014-148441 describes the synthesis of a solid related to an AFX zeolite, specifically a SAPO-56 containing copper usable for NOx reduction. The solid is synthesized and then added to a mixture containing an alcohol and a copper salt, which is then calcined. The copper is thus added after the formation of the SAPO-56 solid, which is related to the structurally AFX zeolite. This exchanged solid appears to exhibit increased resistance to the presence of water.

[0011] Ogura et al. (Bull. Chem. Soc. Jpn. 2018, 91, 355-361) show the very good activity of a copper-exchanged SSZ-16 type zeolite compared to other zeolitic structures, even after hydrothermal aging.

[0012] WO 2017 / 080722 presents the direct synthesis of a copper-containing zeolite. This synthesis requires starting with a structurally FAU zeolite and using a complexing agent TEPA and an M(OH)x element to obtain different types of zeolites, primarily of the CHA type. ANA, ABW, PHI, and GME type zeolites are also produced.

[0013] The applicant discovered that a catalyst based on an AFX structural type zeolite prepared according to a particular rapid synthesis method and at least one transition metal, in particular copper, exhibited interesting performance in the conversion of NOx and selectivity towards N2O. The NOx conversion performance, in particular at low temperature (T<250°C), is notably superior to that obtained with prior art catalysts, such as catalysts based on copper-exchanged AFX structural type zeolite, while maintaining good selectivity towards nitrous oxide N2O. SUMMARY OF THE INVENTION

[0014] The invention relates to a process for preparing a catalyst based on an AFX structural type zeolite and at least one transition metal comprising at least the following steps: (i) A mixture in aqueous medium of at least one source of at least one silicon dioxide (SiO₂), at least one source of at least one aluminum oxide (Al₂O₃), a nitrogenous organic compound R, also called a specific structuring agent, 1,6-bis(methylpiperidinium)hexane dihydroxide, and at least one alkali metal and / or an alkaline earth metal M of valence n, where n is an integer greater than or equal to 1, the reaction mixture having the following molar composition: SiO₂ / Al₂O₃ between 2.00 and 100, preferably between 12 and 40; H₂O / SiO₂ between 5 and 60, preferably between 10 and 40; R / SiO₂ between 0.05 and 0.50, preferably between 0.10 and 0.40; M₂ / nO / SiO₂ between 0.05 and 0.50, preferably between 0.10 and 0.40. between 0.05 to 0.40, preferably between 0.15 and 0.30, where M is sodium, step i) being carried out for a period of time allowing the obtaining of a homogeneous mixture called precursor gel;ii) the hydrothermal treatment of said precursor gel obtained at the end of step i) under autogenous pressure at a temperature between 120°C and 250°C, preferably between 150°C and 230°C, for a period of between 2 and 12 hours, preferably between 2 and 10 hours until said AFX structural type zeolite is formed. iii) at least one ion exchange comprising contacting said solid obtained at the end of the preceding step with a solution comprising at least one species capable of releasing a transition metal, in particular copper, in solution in reactive form under stirring at room temperature for a period of between 1 hour and 2 days, the transition metal being copper and / or iron and the content of transition metal(s) introduced by the ion exchange step iii) being included; between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass relative to the total mass of the final anhydrous catalyst,(iv) a heat treatment advantageously comprising drying the solid obtained in the preceding step (ii) or (iii) at a temperature between 20 and 150°C, preferably between 60 and 100°C, for a period of between 2 and 24 hours, followed by at least one calcination, under air, possibly dry, at a temperature between 450 and 700°C, preferably between 500 and 600°C for a period of between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of air, possibly dry, preferably being between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated.

[0015] At least one source of at least one silicon oxide SiO2, and / or at least one source of at least one aluminium oxide Al2O3 may be at least one structural type FAU zeolite having a molar ratio SiO2 / Al2O3 between 2.00 and 100.

[0016] Steps iii) and iv) can be reversed, and / or possibly repeated.

[0017] Crystalline seeds of a zeolite of structural type AFX may be added to the reaction mixture of step i), preferably in an amount between 0.01 and 10% of the total mass of the sources of said tetravalent (Si) and trivalent (Al) elements in their oxide form (SiO2 and Al2O3) used in the reaction mixture, said crystalline seeds not being taken into account in the total mass of the sources of the tetravalent and trivalent elements.

[0018] Step i) may include a maturation step of the reaction mixture at a temperature between 20 and 80°C, with or without stirring, for a period of between 30 minutes and 24 hours.

[0019] Step iii) of ion exchange is advantageously carried out by bringing the solid into contact with a solution comprising a single species capable of releasing a transition metal or by successively bringing the solid into contact with different solutions each comprising at least one, preferably only one, species capable of releasing a transition metal, preferably the transition metals of the different solutions being different from each other.

[0020] Said at least one transition metal released in the exchange solution of step iii) may be selected from the group formed of the following elements: Fe or Cu and even more preferably said transition metal is Cu.

[0021] The content of transition metal(s) introduced by the ion exchange step iii) is advantageously between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

[0022] The description also relates to the catalyst based on an AFX zeolite and at least one transition metal that can be obtained or directly obtained by the preparation process.

[0023] The transition metal or metals may be selected from the group formed by the following elements: Fe or Cu and even more preferably said transition metal is Cu.

[0024] The total content of transition metals is advantageously between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

[0025] The catalyst comprises copper alone, at a content of between 0.5 and 6% by weight, preferably between 0.5 and 5% by weight, most preferably between 1 and 4% by weight relative to the total mass of the final anhydrous catalyst.

[0026] The catalyst comprises copper in association with at least one other transition metal selected from the group formed by Fe, Nb, Ce, Mn, the copper content of the catalyst being between 0.05 and 2% by mass, preferably 0.5 and 2% by mass, the content of said at least one other transition metal being between 1 and 4% by mass relative to the total mass of the final anhydrous catalyst.

[0027] In yet another embodiment, the catalyst comprises iron in association with another metal chosen from the group formed by Cu, Nb, Ce, Mn, the iron content being between 0.05 and 2% by mass, preferably between 0.5 and 2% by mass, the content of said other transition metal being between 1 and 4% by mass, relative to the total mass of the final anhydrous catalyst.

[0028] The description also relates to a process for the selective reduction of NOx by a reducing agent such as NH3 or H2 employing a catalyst as described above or a catalyst that can be obtained or is directly obtained by the preparation process.

[0029] The catalyst can be shaped by deposition as a coating, on a honeycomb structure or a plate structure.

[0030] The honeycomb structure can be formed of parallel channels open at both ends or can include porous filtering walls in which adjacent parallel channels are alternately blocked on either side of the channels.

[0031] The quantity of catalyst deposited on said structure is advantageously between 50 and 180 g / L for filter structures and between 80 and 200 g / L for structures with open channels.

[0032] The catalyst can be associated with a binder such as cerine, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a mixed oxide of the cerine-zirconia type, a tungsten oxide and / or a spinel to be shaped by deposition as a coating.

[0033] The said coating can be associated with another coating having the capacity to adsorb pollutants in particular NOx, to reduce pollutants in particular NOx or to promote the oxidation of pollutants.

[0034] The said catalyst may be in extruded form, containing up to 100% of said catalyst.

[0035] The structure coated by said catalyst or obtained by extrusion of said catalyst can be integrated into an exhaust line of an internal combustion engine. LIST OF FIGURES

[0036] There Figure 1 represents the chemical formula of the nitrogenous organic compound R, which is the structuring agent used in the synthesis process according to the invention. Figure 2 represents the X-ray diffraction patterns of AFX structural-type copper-containing zeolites obtained according to examples 2 to 5. Figure 3represents the conversion C in % obtained during a catalytic test of the reduction of nitrogen oxides (NOx) by ammonia (NH3) in the presence of oxygen (O2) under Standard SCR conditions as a function of temperature T in °C for a catalyst according to example 2 (CuAFX, according to the invention, curve symbolized by the diamonds), a catalyst according to example 3 (CuAFX780, according to the invention, curve symbolized by the triangles), a catalyst according to example 4 (CuAFX720, according to the invention, curve symbolized by the squares), a catalyst according to example 5 (CuAFX600, according to the invention, curve symbolized by the circles) and a catalyst according to example 6 (CuSSZ16, comparative, curve symbolized by the crosses).

[0037] Other features and advantages of the synthesis process according to the invention, of the catalyst and of the use, will become apparent from the following description of non-limiting examples of embodiments, with reference to the figures attached and described below. DESCRIPTION OF IMPLEMENTATION METHODS

[0038] The present invention relates to a process for preparing a catalyst comprising an AFX structural type zeolite and at least one transition metal, comprising at least the following steps: (i) A mixture in aqueous medium of at least one source of at least one silicon dioxide (SiO₂), at least one source of at least one aluminium oxide (Al₂O₃), or at least one structural zeolite (FAU) having a molar ratio of SiO₂ / Al₂O₃ between 2.00 and 100, of a nitrogenous organic compound R, also called a specific structuring agent, 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one alkali metal and / or an alkaline earth metal M of valence n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition: SiO₂ / Al₂O₃ between 2 and 100, preferably between 12 and 40; H₂O / SiO₂ between 5 and 60, preferably between 10 and 40; R / SiO₂ 2 between 0.05 and 0.50, preferably between 0.10 and 0.40 M 2 / n O / SiO 2 between 0.05 and 0.40, preferably between 0.15 and 0.30,in which M is sodium, step i) being carried out for a period enabling the obtaining of a homogeneous mixture called precursor gel; ii) the hydrothermal treatment of said precursor gel obtained at the end of step i) under autogenous pressure at a temperature between 120°C and 250°C, preferably between 150°C and 230°C, for a period between 2 and 12 hours, preferably between 2 and 10 hours, until said structural type AFX zeolite is formed. iii) at least one ion exchange comprising bringing said solid obtained at the end of step ii) into contact with a solution comprising at least one species capable of releasing a transition metal, in particular copper, in solution in reactive form under stirring at room temperature for a period of between 1 hour and 2 days, the transition metal being copper and / or iron and the content of transition metal(s) introduced by the ion exchange step iii) being included, between 0.5 to 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass relative to the total mass of the final anhydrous catalyst; (iv) a heat treatment advantageously comprising drying the solid obtained in the preceding step (ii) or (iii) at a temperature between 20 and 150°C, preferably between 60 and 100°C, for a period of between 2 and 24 hours, followed by at least one calcination, under air, possibly dry, at a temperature between 450 and 700°C, preferably between 500 and 600°C for a period of between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of air, possibly dry, preferably being between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated.

[0039] Steps iii) and iv) can be reversed, and / or possibly repeated.

[0040] This description also relates to the catalyst comprising an AFX structural type zeolite and at least one transition metal that can be obtained or directly obtained by the process described above.

[0041] The description finally relates to the use of a catalyst in a catalytic selective reduction process of NOx in the presence of a reducing agent. The catalyst (not claimed)

[0042] The catalyst comprises at least one AFX type zeolite, and at least one additional transition metal, preferably copper.

[0043] The transition metal(s) included in the catalyst is / are selected from the elements in groups 3 to 12 of the periodic table, including the lanthanides. In particular, the transition metal(s) included in the catalyst is / are selected from the group consisting of the following elements: FeCu.

[0044] Preferably, the catalyst comprises copper, alone or combined with Fe.

[0045] The total content of transition metals is advantageously between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, and even more preferably between 1 and 4% by mass, relative to the total mass of the final catalyst in its anhydrous form.

[0046] For catalysts which contain only copper as a transition metal, the content is advantageously between 0.5 and 6%, preferably between 0.5 and 5%, and more preferably between 1 and 4% by weight relative to the total mass of the final anhydrous catalyst.

[0047] For catalysts comprising copper and another element such as, preferably, Fe, Nb, Ce, Mn, the copper content of the catalyst is between 0.05 and 2% by mass, preferably 0.5 and 2% by mass, while that of the other transition metal is preferably between 1 and 4% by mass, the transition metal contents being given as mass percentages relative to the total mass of the final dry catalyst.

[0048] For catalysts which contain only iron as a transition metal, the iron content is between 0.5 and 4% and preferably between 1.5 and 3.5% relative to the total mass of the final anhydrous catalyst.

[0049] For catalysts comprising iron and another element such as, preferably, Cu, Nb, Ce, Mn, the iron content of the catalyst is between 0.05 and 2% by mass, preferably between 0.5 and 2% by mass, while that of the other transition metal is preferably between 1 and 4% by mass, the transition metal contents being given as mass percentages relative to the total mass of the final dry catalyst.

[0050] The catalyst may also include other elements, such as alkali and / or alkaline earth metals, for example sodium, obtained in particular from the synthesis, especially from the compounds of the reaction medium of step i) of the process of preparing said catalyst. Catalyst preparation process Step i) of mixing

[0051] This step involves mixing in aqueous medium at least one source of at least one silicon dioxide SiO2, at least one source of at least one aluminium oxide Al2O3, and / or at least one structural type zeolite FAU having a molar ratio SiO2 / Al2O3 between 2.00 and 100, a nitrogenous organic compound R, also called a specific structuring agent, 1,6-bis(methylpiperidinium)hexane dihydroxide, at least one alkali metal and / or an alkaline earth metal M of valence n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition: SiO2 / Al2O3 between 2 and 100, preferably between 12 and 40; H2O / SiO2 between 5 and 60, preferably between 10 and 40; R / SiO2 between 0.05 and 0.50, preferably between 0.10 and 0.40; M2 / nO / SiO2 between 0.05 and 0.40, preferably between 0.15 and 0.30, wherein M is sodium, step i) being carried out for a duration enabling the obtaining of a homogeneous mixture called precursor gel.

[0052] In the molar composition of the reaction mixture above and in the overall description: SiO2 denotes the molar quantity of the tetravalent element silicon (Si) expressed as oxide, and Al2O3 denotes the molar quantity of the trivalent element aluminium (Al) expressed as oxide, H2O the molar quantity of water present in the reaction mixture, R the molar quantity of said nitrogenous organic compound, M2 / nO the molar quantity expressed as oxide of M2 / nO by the alkali metal and / or alkaline earth metal source.

[0053] According to the invention, at least one source of SiO₂ oxide is incorporated into the mixture for carrying out step (i) of the preparation process. The silicon source can be any of the sources commonly used for the synthesis of zeolites, for example, powdered silica, silicic acid, colloidal silica, dissolved silica, or tetraethoxysilane (TEOS). Among the powdered silicas, precipitated silicas may be used, in particular those obtained by precipitation from an alkali metal silicate solution, fumed silicas, for example, "CAB-O-SIL," and silica gels. Colloidal silicas with different particle sizes may be used, for example, with an average equivalent diameter between 10 and 15 nm or between 40 and 50 nm, such as those marketed under registered trademarks such as "LUDOX." Preferably, the silicon source is the LUDOX HS-40.As a source of SiO2 oxide, one can also use at least one FAU structural type zeolite having a SiO2 / Al2O3 molar ratio between 2.00 and 100 alone or in mixture with other SiO2 sources.

[0054] The aluminum source is preferably aluminum hydroxide or an aluminum salt, for example, chloride, nitrate, or sulfate, sodium aluminate, aluminum alkoxide, or alumina itself, preferably in a hydrated or hydratable form, such as colloidal alumina, pseudoboehmite, gamma alumina, or alpha or beta trihydrate. Mixtures of the sources mentioned above may also be used. As an Al₂O₃ oxide source, at least one FAU-type zeolite with a SiO₂ / Al₂O₃ molar ratio between 2.00 and 100 may also be used, either alone or in a mixture with other Al₂O₃ sources.

[0055] According to the invention, at least one source of silica, and / or at least one source of aluminium may also be at least one structural FAU type zeolite having a SiO2 / Al2O3 molar ratio between 2.00 and 100.

[0056] According to the invention, R is a nitrogenous organic compound, 1,6-bis(methylpiperidinium)hexane dihydroxide, said compound being incorporated into the reaction mixture for the implementation of step (i), as an organic structuring agent.

[0057] According to the invention, at least one source of at least one alkali and / or alkaline earth metal M of valence n is used in the reaction mixture of step i), n being an integer greater than or equal to 1, M preferably being chosen from lithium, potassium, sodium, magnesium, and calcium, and mixtures of at least two of these metals. Most preferably, M is sodium.

[0058] Preferably, the source of at least one alkali and / or alkaline earth metal M is sodium hydroxide.

[0059] It may be advantageous to add nuclei of an AFX structural zeolite to the reaction mixture during step i) of the process of the invention in order to reduce the time required for the formation of AFX structural zeolite crystals and / or the total crystallization time. These nuclei also promote the formation of said AFX structural zeolite at the expense of impurities. Such nuclei include crystalline solids, in particular crystals of an AFX structural zeolite. The nuclei are generally added in a proportion of between 0.01 and 10% of the total mass of the sources of said tetravalent, silicon, and trivalent, aluminum, oxide form used in the reaction mixture, said nuclei not being included in the total mass of the sources of the tetravalent and trivalent elements.The said germs are also not taken into account in determining the composition of the reaction mixture and / or the gel, defined further on, that is to say in the determination of the different molar ratios of the composition of the reaction mixture.

[0060] Step i) of mixing is carried out until a homogeneous mixture is obtained, preferably for a period greater than or equal to 10 minutes, preferably under agitation by any system known to the person skilled in the art with low or high shear rate.

[0061] At the end of step i) a homogeneous precursor gel is obtained.

[0062] It may be advantageous to mature the reaction mixture before hydrothermal crystallization during step i) of the process of the invention in order to control the crystal size of an AFX structural zeolite. This maturation also promotes the formation of said AFX structural zeolite at the expense of impurities. The maturation of the reaction mixture during step i) of the process of the invention can be carried out at room temperature or at a temperature between 20 and 80°C, with or without stirring, for a duration advantageously between 30 minutes and 24 hours. Step ii) of hydrothermal treatment

[0063] In accordance with step ii) of the process according to the invention, the precursor gel obtained at the end of step i) is subjected to hydrothermal treatment, preferably carried out at a temperature between 120°C and 250°C for a period of between 2 and 12 hours, until said structural type AFX zeolite (or "crystallized solid") is formed.

[0064] The precursor gel is advantageously placed under hydrothermal conditions under autogenic reaction pressure, possibly by adding gas, for example nitrogen, at a temperature preferably between 120°C and 250°C, preferably between 150°C and 230°C, until complete crystallization of a zeolite of structural type AFX.

[0065] The time required to achieve crystallization varies between 2 and 12 hours, preferably between 2 and 10 hours, and more preferably between 2 and 8 hours.

[0066] The reaction is generally carried out with or without agitation, preferably with agitation. Any system known to those skilled in the art can be used as an agitation system, for example, inclined blades with counter-blades, stirring turbines, or Archimedes screws. Step iii) of exchange

[0067] The catalyst preparation process according to the invention comprises at least one ion exchange step, comprising contacting the crystalline solid obtained at the end of the preceding step, i.e., the AFX zeolite obtained at the end of step ii) or the dried and calcined AFX zeolite obtained at the end of step iv) in the preferred case where steps iii) and iv) are reversed, with at least one solution comprising at least one species capable of releasing a transition metal, preferably copper, in solution in reactive form, under stirring at room temperature for a period of between 1 hour and 2 days, advantageously for a period of between 6 and 12 hours, the transition metal being copper and / or iron and the content of transition metal(s) introduced by the ion exchange step iii) being between 0.5 and 6% by mass, preferably between 0.5 and 5% by mass, more preferably between 1 and 4% by mass relative to the total mass of the final anhydrous catalyst,the concentration of said species capable of releasing the transition metal in said solution being a function of the quantity of transition metal that one wishes to incorporate into said crystallized solid.

[0068] It is also advantageous to obtain the protonated form of the AFX structural type zeolite after step ii). This hydrogen form can be obtained by carrying out an ion exchange with an acid, in particular a strong mineral acid such as hydrochloric, sulfuric or nitric acid, or with a compound such as ammonium chloride, sulfate or nitrate, before the ion exchange with the transition metal(s).

[0069] The transition metal released into the exchange solution is selected from the group consisting of the following elements: Fe, Cu, preferably Cu.

[0070] According to the invention, a "species capable of releasing a transition metal" is understood to mean a species capable of dissociating in aqueous media, such as sulfates, nitrates, chlorides, oxalates, organometallic complexes of a transition metal, or mixtures thereof. Preferably, the species capable of releasing a transition metal is a sulfate or nitrate of said transition metal.

[0071] According to the invention, the solution with which the crystallized solid or the dried and calcined crystallized solid is brought into contact comprises at least one species capable of releasing a transition metal, preferably a single species capable of releasing a transition metal, preferably iron or copper, preferably copper.

[0072] Advantageously, the catalyst preparation process according to the invention includes a step iii) of ion exchange by contacting the crystallized solid with a solution comprising a species capable of releasing a transition metal or by successively contacting the solid with several solutions each comprising a species capable of releasing a transition metal, the different solutions comprising different species capable of releasing a transition metal.

[0073] At the end of the exchange, the solid obtained is advantageously filtered, washed and then dried to obtain said catalyst in powder form.

[0074] The total amount of transition metal, preferably copper, contained in said final catalyst is between 0.5 and 6% by mass relative to the total mass of the catalyst in its anhydrous form.

[0075] The catalyst is prepared by a process comprising an ion exchange step (iii), in which the solid or dried and calcined solid is contacted with a solution containing a species capable of releasing copper in solution in a reactive form. Advantageously, the total amount of copper contained in the final catalyst, i.e., at the end of the preparation process, is between 0.5 and 6%, preferably between 1 and 4% by mass, all percentages being mass percentages relative to the total mass of the final catalyst in its anhydrous form, obtained at the end of the preparation process. Stage iv) heat treatment

[0076] The preparation process according to the invention comprises a heat treatment step (iv) carried out after the preceding step, i.e., after hydrothermal treatment step (ii) or after ion exchange step (iii), preferably after ion exchange step (iii). Step (iii) of the preparation process may advantageously be interchanged with step (iv). Each of the two steps (iii) and (iv) may also optionally be repeated.

[0077] Heat treatment step iv) comprises drying the solid at a temperature between 20 and 150°C, preferably between 60 and 100°C, advantageously for a period of 2 to 24 hours, followed by at least one calcination, under air, possibly dry, at a temperature advantageously between 450 and 700°C, preferably between 500 and 600°C, for a period of 2 to 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of the air, possibly dry, preferably being between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated. The calcination may be preceded by a gradual temperature increase.

[0078] The catalyst obtained at the end of step iv) of heat treatment is free of any organic species, in particular free of the organic structuring agent R.

[0079] In particular, the catalyst obtained by a process comprising at least steps i), ii), iii), and iv) previously described exhibits improved properties for the conversion of NOx Catalyst Characterization

[0080] The catalyst comprises a zeolite with an AFX structure according to the International Zeolite Association (IZA) classification, exchanged by at least one transition metal. This structure is characterized by X-ray diffraction (XRD).

[0081] The X-ray diffraction (XRD) pattern is obtained by X-ray crystallography using a diffractometer and the classical powder diffraction method with copper Kα1 radiation (λ = 1.5406 Å). From the position of the diffraction peaks, represented by the angle 2θ, the characteristic interplanar spacings dhkl of the sample are calculated using Bragg's law. The measurement error Δ(dhkl) is calculated using Bragg's law as a function of the absolute error Δ(2θ) assigned to the measurement of 2θ. An absolute error Δ(2θ) of ±0.02° is commonly accepted. The relative intensity Irel assigned to each value of dhkl is measured from the height of the corresponding diffraction peak. Comparing the diffractogram with the records in the ICDD (International Centre for Diffraction Data) database using software such as DIFFRACT.SUITE also allows us to identify the crystalline phases present in the material obtained.

[0082] The qualitative and quantitative analysis of the chemical species present in the materials obtained is performed by X-ray fluorescence (XRF) spectrometry. This is a chemical analysis technique that utilizes a physical property of matter: X-ray fluorescence. The spectrum of X-rays emitted by the material is characteristic of the sample's composition; by analyzing this spectrum, the elemental composition, that is, the mass concentrations of the elements, can be deduced.

[0083] The loss on ignition (LOI) of the catalyst obtained after the drying step (and before calcination) or after the calcination step of step iv) of the process is generally between 5 and 18% by weight. The loss on ignition of a sample, designated by the acronym LOI, corresponds to the difference in mass of the sample before and after heat treatment at 1000°C for 2 hours. It is expressed as a percentage, corresponding to the percentage of mass loss. The loss on ignition generally corresponds to the loss of solvent (such as water) contained in the solid, but also to the removal of organic compounds contained in the solid mineral constituents. Selective reduction process of NOx by a reducing agent such as NH3 or H2 using the catalyst (unclaimed)

[0084] The description also relates to the use of the catalyst, directly prepared or capable of being prepared by the process described above for the selective reduction of NOx by a reducing agent such as NH3 or H2, advantageously shaped by deposition as a coating ("washcoat" according to Anglo-Saxon terminology) on a honeycomb structure mainly for mobile applications or a plate structure which is found particularly for stationary applications.

[0085] The honeycomb structure is formed of parallel channels open at both ends (flow-through) or has porous filter walls, in which case adjacent parallel channels are alternately blocked on either side to force the gas flow through the wall (wall-flow monolith). This coated honeycomb structure constitutes a catalytic block. The structure can be composed of cordierite, silicon carbide (SiC), aluminum titanate (AlTi), alpha alumina, mullite, or any other material with a porosity between 30 and 70%. The structure can be made of sheet metal, stainless steel containing chromium and aluminum, or FeCrAl steel.

[0086] The amount of catalyst deposited on said structure is between 50 and 180 g / L for filter structures and between 80 and 200 g / L for structures with open channels.

[0087] The coating itself (the "washcoat") comprises the catalyst, advantageously combined with a binder such as cerine, zirconium oxide, alumina, non-zeolitic silica-alumina, titanium oxide, a mixed oxide of the cerine-zircone type, a tungsten oxide, or a spinel. This coating is advantageously applied to the structure by a deposition method (washcoating) which consists of dipping the monolith into a slurry of catalyst powder in a solvent, preferably water, and potentially binders, metal oxides, stabilizers, or other promoters. This dipping step can be repeated until the desired amount of coating is achieved. In some cases, the slurry can also be sprayed into the monolith. Once the coating is deposited, the monolith is calcined at a temperature of 300 to 600°C for 1 to 10 hours.

[0088] The said structure can be coated with one or more coatings. The coating including the catalyst is advantageously associated with, that is to say covers or is covered by, another coating having the capacity to adsorb pollutants in particular NOx, to reduce pollutants in particular NOx or to promote the oxidation of pollutants, in particular that of ammonia.

[0089] Another possibility is to put the catalyst in extruded form. In this case, the resulting structure can contain up to 100% catalyst.

[0090] The structure coated with the catalyst is advantageously integrated into the exhaust system of an internal combustion engine operating primarily under lean conditions, i.e., with an excess of air relative to the stoichiometric ratio of the combustion reaction, as is the case for diesel engines, for example. Under these engine operating conditions, the exhaust gases contain, in particular, the following pollutants: soot, unburned hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Upstream of the structure coated with the catalyst, an oxidation catalyst can be placed, whose function is to oxidize HC and CO, as well as a filter to remove soot from the exhaust gases. The function of the coated structure is to remove NOx, and its operating range is between 100 and 900°C, preferably between 200 and 500°C. Advantages of the invention

[0091] The catalyst, based on a rapidly synthesized AFX structural zeolite and at least one transition metal, particularly copper, exhibits improved properties compared to prior art catalysts. Specifically, the catalyst enables lower initiation temperatures for the NOx conversion reaction and improved NOx conversion across the entire operating temperature range (150–600°C), while maintaining good N₂O selectivity. It also demonstrates better resistance to hydrothermal aging, ensuring high performance even after this aging process. EXAMPLES Example 1: preparation of 1,6-bis(methylpiperidinium)hexane dihydroxide (structuring agent R).

[0092] 50 g of 1,6-dibromohexane (0.20 mol, 99%, Alfa Aesar) are added to a 1 L flask containing 50 g of N-methylpiperidine (0.51 mol, 99%, Alfa Aesar) and 200 mL of ethanol. The reaction mixture is stirred and refluxed for 5 hours. The mixture is then cooled to room temperature and filtered. The mixture is poured into 300 mL of cold diethyl ether, and the resulting precipitate is filtered and washed with 100 mL of diethyl ether. The solid obtained is recrystallized from an ethanol / ether mixture. The resulting solid is dried under vacuum for 12 hours. 71 g of a white solid are obtained (80% yield).

[0093] The product has the expected 1<H NMR spectrum. 1<H NMR (D 2 O, ppm / TMS): 1.27 (4H,m); 1.48 (4H,m); 1.61 (4H,m); 1.70 (8H,m); 2.85 (6H,s); 3.16 (12H,m).

[0094] 18.9 g of Ag₂O (0.08 mol, 99%, Aldrich) are added to a 250 mL Teflon beaker containing 30 g of the prepared structuring agent 1,6-bis(methylpiperidinium)hexane dibromide (0.07 mol) and 100 mL of deionized water. The reaction mixture is stirred in the dark for 12 hours. The mixture is then filtered. The resulting filtrate consists of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide. The concentration of this species is determined by proton NMR using formic acid as a standard. Example 2: preparation of a structural type AFX zeolite according to the invention with 3% Cu Preparation of AFX zeolite

[0095] 467.1 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36 wt.) prepared according to Example 1 were mixed with 4.3 g of deionized water. 19.72 g of sodium hydroxide (solid, 98 wt. purity, Aldrich) were added to the mixture, and the resulting preparation was stirred for 10 minutes. Subsequently, 15.56 g of sodium aluminate (53.17% Al₂O₃, Strem Chemicals) were incorporated, and the synthesis gel was stirred for 15 minutes. In the end, 243.38 g of colloidal silica (Ludox HS40, 40% SiO2 by weight, Grace) and 9.76 g of AFX zeolite nuclei obtained by a method known to those skilled in the art were incorporated into the synthesis mixture with stirring. The molar composition of the mixture, excluding the AFX zeolite nuclei, is as follows: 100 SiO2:5 Al2O3:16.7 R:22.4 Na2O:1836 H2O, resulting in a SiO2 / Al2O3 ratio of 20.The precursor gel, after homogenization, is then transferred to an autoclave. The autoclave is closed and heated at a rate of 5°C / min to 150°C for 4 hours under autogenous pressure and with agitation at 200 rpm using a four-bladed, inclined impeller. The resulting crystallized product is filtered, washed with deionized water, and then dried overnight at 100°C. The loss on ignition is 14%. The solid is then introduced into a muffle furnace where a calcination step is carried out: the calcination cycle consists of a rate of 1.5°C / min to 200°C, a holding time at 200°C for 2 hours, a rate of 1°C / min to 550°C, followed by a holding time at 550°C for 12 hours, and then a return to ambient temperature.

[0096] The calcined solid product was analyzed by X-ray diffraction and identified as a structurally type AFX zeolite (ICDD datasheet, PDF 04-011-1869) with a purity greater than 99.8%. The product has a SiO₂ / Al₂O₃ molar ratio of 10.2 as determined by X-ray diffraction.

[0097] The calcined AFX zeolite is then contacted with a 3 molar NH₄NO₃ solution for 1 hour under stirring at 80°C. The ratio of the volume of NH₄NO₃ solution to the mass of solid is 10. The resulting solid is filtered and washed, and the exchange procedure is repeated twice more under the same conditions. The final solid is separated, washed, and dried for 12 hours at 100°C. XRD analysis shows that the product obtained is a pure AFX structural-type zeolite.

[0098] AFX zeolite in ammoniacal form is treated under an airflow at 550°C for 8 hours with a temperature ramp rate of 1°C / min. The resulting product is AFX zeolite in protonated form. Ion exchange with Cu

[0099] The calcined protonated AFX zeolite is contacted with a [Cu(NH3)4][NO3)2 solution for 12 hours with stirring at room temperature. The final solid is separated, washed, and dried for 12 hours at 100°C.

[0100] The exchanged Cu-AFX solid obtained after contact with the [Cu(NH3)4][NO3)2 solution is calcined under airflow at 550°C for 8 hours.

[0101] The calcined solid product was analyzed by X-ray diffraction and identified as a structurally type AFX zeolite (ICDD datasheet, PDF 04-011-1869). The diffraction pattern obtained for this solid is shown in the diagram. Figure 2 .

[0102] The product has a SiO2 / Al2O3 molar ratio of 10.2 and a Cu mass percentage of 3% as determined by FX.

[0103] The resulting catalyst is denoted CuAFX. Example 3: Preparation of a structural-type AFX zeolite according to the invention 3% Cu Preparation of the AFX zeolite

[0104] 29.3 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36 wt.) prepared according to Example 1 are mixed with 41.73 g of deionized water, with stirring and at room temperature. 0.764 g of sodium hydroxide (98 wt., Aldrich) are dissolved in the preceding mixture with stirring and at room temperature. Subsequently, 0.675 g of amorphous aluminum hydroxide gel (Al(OH)₃ amorphous gel, 58.55 wt. of Al₂O₃, Merck) are incorporated into the synthesis mixture, which is then stirred for half an hour at room temperature. As soon as the suspension obtained is homogeneous, 7.56 g of a structural type FAU zeolite (CBV780, SiO 2 / Al 2 O 3 = 98.22, Zeolyst, PAF = 8.52%) is added and the suspension obtained is kept under stirring for 30 minutes at room temperature.To promote the formation of an AFX structural zeolite, 0.614 g of AFX structural zeolite seeds (10% by mass of CBV780 zeolite) are added to the synthesis mixture, which is stirred for 5 minutes. The reaction mixture then undergoes a maturation step for 24 hours at room temperature with stirring (200 rpm). The molar composition of the precursor gel is as follows: 1 SiO₂: 0.05 Al₂O₃: 0.167 R: 0.093 Na₂O: 36.73 H₂O, resulting in a SiO₂ / Al₂O₃ ratio of 20. The precursor gel is then transferred to a 160 mL stainless steel reactor equipped with a four-bladed stirring system. The reactor is closed, then heated for 5 hours under autogenous pressure with a temperature rise of 5°C / min up to 180°C under agitation at 200 rpm to allow the crystallization of the AFX structural type zeolite.The crystallized product obtained is filtered, washed with deionized water, then dried overnight at 100°C. The loss on ignition of the dried solid is 14.69%.

[0105] The solid is then introduced into a muffle furnace where a calcination step is carried out: the calcination cycle includes a temperature rise of 1.5°C / min up to 200°C, a plateau at 200°C maintained for 2 hours, a rise of 1°C / min up to 550°C followed by a plateau at 550°C maintained for 12 hours and then a return to ambient temperature.

[0106] The calcined solid product was analyzed by X-ray diffraction and identified as a structurally type AFX zeolite (ICDD datasheet, PDF 04-011-1869) with a purity greater than 99% by weight. The product has a SiO₂ / Al₂O₃ molar ratio of 14.05 as determined by X-ray diffraction.

[0107] The calcined AFX zeolite is then contacted with a 3-molar NH₄NO₃ solution for 1 hour under stirring at 80°C. The ratio of the NH₄NO₃ solution volume to the solid mass is 10. The resulting solid is filtered and washed, and the exchange procedure is repeated twice more under the same conditions. The final solid is separated, washed, and dried for 12 hours at 100°C. XRD analysis shows that the product obtained is a pure AFX structural-type zeolite.

[0108] AFX zeolite in ammoniacal form is treated under an airflow at 550°C for 8 hours with a temperature ramp rate of 1°C / min. The resulting product is AFX zeolite in protonated form. Ion exchange with Cu

[0109] The calcined protonated AFX zeolite is contacted with a [Cu(NH3)4][NO3)2 solution for 12 hours with stirring at room temperature. The final solid is separated, washed, and dried for 12 hours at 100°C.

[0110] The exchanged Cu-AFX solid obtained after contact with the [Cu(NH3)4][NO3)2 solution is calcined under airflow at 550°C for 8 hours.

[0111] The calcined solid product was analyzed by X-ray diffraction and identified as a structurally type AFX zeolite (ICDD datasheet, PDF 04-011-1869). The diffraction pattern obtained for this solid is shown in the diagram. Figure 2 .

[0112] The product has a SiO2 / Al2O3 molar ratio of 14.05 and a Cu mass percentage of 3% as determined by FX.

[0113] The resulting catalyst is designated CuAFX780. Example 4: preparation of a structural type AFX zeolite according to the invention 3% Cu Preparation of AFX zeolite

[0114] 33.37 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36 wt.) prepared according to Example 1 are mixed with 37.15 g of deionized water, with stirring and at room temperature. 1.72 g of sodium hydroxide (98 wt., Aldrich) are dissolved in the preceding mixture with stirring and at room temperature. As soon as the resulting suspension is homogeneous, 7.79 g of a structurally type FAU zeolite (CBV720, SiO₂ / Al₂O₃ = 33.52, Zeolyst, PAF = 6.63%) are added, and the resulting suspension is stirred for 30 minutes at room temperature. To promote the formation of an AFX structural zeolite, 0.646 g of AFX structural zeolite seeds (10% by mass of CBV720 zeolite) are added to the synthesis mixture and stirred for 5 minutes. The reaction mixture then undergoes a maturation step for 24 hours at room temperature with stirring (200 rpm).The molar composition of the precursor gel is as follows: 1 SiO₂: 0.0298 Al₂O₃: 0.18 R: 0.20 Na₂O: 34 H₂O, resulting in a SiO₂ / Al₂O₃ ratio of 33.55. After homogenization, the precursor gel is transferred to a 160 mL stainless steel reactor equipped with a four-bladed stirring system. The reactor is closed and then heated for 5 hours under autogenous pressure, increasing the temperature at a rate of 5°C / min up to 180°C with stirring at 200 rpm to allow crystallization of the AFX structural zeolite. The resulting crystallized product is filtered, washed with deionized water, and then dried overnight at 100°C. The loss on ignition of the dried solid is 14.82%.The solid is then introduced into a muffle furnace where a calcination step is carried out: the calcination cycle includes a temperature rise of 1.5°C / min up to 200°C, a plateau at 200°C maintained for 2 hours, a rise of 1°C / min up to 550°C followed by a plateau at 550°C maintained for 12 hours and then a return to ambient temperature.

[0115] The calcined solid product was analyzed by X-ray diffraction and identified as a structurally type AFX zeolite (ICDD datasheet, PDF 04-011-1869) with a purity greater than 99% by weight. The product has a SiO₂ / Al₂O₃ molar ratio of 11.42 as determined by X-ray diffraction.

[0116] The calcined AFX zeolite is then contacted with a 3-molar NH₄NO₃ solution for 1 hour under stirring at 80°C. The ratio of the NH₄NO₃ solution volume to the solid mass is 10. The resulting solid is filtered and washed, and the exchange procedure is repeated twice more under the same conditions. The final solid is separated, washed, and dried for 12 hours at 100°C. XRD analysis shows that the product obtained is a pure AFX structural-type zeolite.

[0117] AFX zeolite in ammoniacal form is treated under an airflow at 550°C for 8 hours with a temperature ramp rate of 1°C / min. The resulting product is AFX zeolite in protonated form. Ion exchange with Cu

[0118] The calcined protonated AFX zeolite is contacted with a [Cu(NH3)4][NO3)2 solution for 12 hours with stirring at room temperature. The final solid is separated, washed, and dried for 12 hours at 100°C.

[0119] The exchanged Cu-AFX solid obtained after contact with the [Cu(NH3)4][NO3)2 solution is calcined under airflow at 550°C for 8 hours.

[0120] The calcined solid product was analyzed by X-ray diffraction and identified as a structurally type AFX zeolite (ICDD datasheet, PDF 04-011-1869). The diffraction pattern obtained for this solid is shown in the diagram. Figure 2 .

[0121] The product has a SiO2 / Al2O3 molar ratio of 11.42 and a Cu mass percentage of 3% as determined by FX.

[0122] The resulting catalyst is designated CuAFX720. Example 5: preparation of a structural type AFX zeolite according to the invention 3% Cu Preparation of AFX zeolite

[0123] 28.35 g of an aqueous solution of 1,6-bis(methylpiperidinium)hexane dihydroxide (18.36 wt.) prepared according to Example 1 are mixed with 41.22 g of deionized water, with stirring, at room temperature. 1.26 g of sodium hydroxide (98 wt., Aldrich) are dissolved in the preceding mixture with stirring, at room temperature. Then, 5.74 g of Aerosil 380 silica (100 wt., Degussa) are added in small portions with stirring. As soon as the resulting suspension is homogeneous, 3.43 g of FAU structural-type zeolite (CBV600 Zeolyst, SiO₂ / Al₂O₃ = 5.48, PAF = 12.65%) are added, and the resulting suspension is stirred for 30 minutes at room temperature. Next, the reaction mixture undergoes a maturation step for 2 hours at room temperature under stirring (350 rpm).The resulting precursor gel has the following molar composition: 1 SiO₂: 0.05 Al₂O₃: 0.125 R: 0.12 Na₂O: 27.55 H₂O, resulting in a SiO₂ / Al₂O₃ ratio of 20. 0.79 g of AFX structural zeolite seeds (8.7% by mass of anhydrous CBV600 zeolite and Aerosil 380 silica) are then stirred into the precursor gel. The precursor gel containing the AFX zeolite seeds is then transferred to a 160 mL stainless steel reactor equipped with a four-bladed stirring system. The reactor is closed and then heated for 7 hours under autogenous pressure, with a temperature increase of 5°C / min up to 190°C, while stirring at 200 rpm to allow the crystallization of the AFX structural zeolite. The resulting solid is filtered, washed with deionized water, and then dried overnight at 100°C. The loss on ignition of the dried solid is 12.6%.

[0124] The solid is then introduced into a muffle furnace where a calcination step is carried out: the calcination cycle includes a temperature rise of 1.5°C / min up to 200°C, a plateau at 200°C maintained for 2 hours, a rise of 1°C / min up to 580°C followed by a plateau at 580°C maintained for 10 hours and then a return to ambient temperature.

[0125] The calcined solid product was analyzed by X-ray diffraction and identified as a structural-type zeolite of AFX (ICDD datasheet, PDF 04-011-1869) with a purity greater than 97% by weight. The product has a SiO₂ / Al₂O₃ molar ratio of 11.2, as determined by X-ray diffraction.

[0126] The calcined AFX zeolite is then contacted with a 3-molar NH₄NO₃ solution for 1 hour under stirring at 80°C. The ratio of the NH₄NO₃ solution volume to the solid mass is 10. The resulting solid is filtered and washed, and the exchange procedure is repeated twice more under the same conditions. The final solid is separated, washed, and dried for 12 hours at 100°C. XRD analysis shows that the product obtained is a pure AFX structural-type zeolite.

[0127] AFX zeolite in ammoniacal form is treated under an airflow at 550°C for 8 hours with a temperature ramp rate of 1°C / min. The resulting product is AFX zeolite in protonated form. Ion exchange with Cu

[0128] The calcined protonated AFX zeolite is contacted with a [Cu(NH3)4][NO3)2 solution for 12 hours with stirring at room temperature. The final solid is separated, washed, and dried for 12 hours at 100°C.

[0129] The exchanged Cu-AFX solid obtained after contact with the [Cu(NH3)4][NO3)2 solution is calcined under airflow at 550°C for 8 hours.

[0130] The calcined solid product was analyzed by X-ray diffraction and identified as a structurally type AFX zeolite (ICDD datasheet, PDF 04-011-1869). The diffraction pattern obtained for this solid is shown in the diagram. Figure 2 .

[0131] The product has a SiO2 / Al2O3 molar ratio of 11.2 and a Cu mass percentage of 3% as determined by FX.

[0132] The resulting catalyst is designated CuAFX600. Example 6

[0133] In this example, a copper-exchanged SSZ-16 zeolite is synthesized according to the prior art. In this example, the copper is introduced by ion exchange. Preparation of SSZ-16 zeolite

[0134] 17.32 g of sodium hydroxide are dissolved in 582.30 g of deionized water, with stirring (300 rpm) at room temperature. 197.10 g of sodium silicate are added to this solution and the mixture is homogenized with stirring (300 rpm) at room temperature. Next, 9.95 g of NaY CBV100 zeolite are added with stirring (300 rpm) and this process is continued until the zeolite is dissolved. Finally, 43.67 g of the structuring agent DABCO-C4 are dissolved in the resulting solution and homogenized with stirring (450 rpm) for 30 minutes at room temperature.

[0135] The reaction mixture has the following molar composition: 100 SiO2: 1.67 Al2O3: 50 Na2O: 10 DABCO-C4: 4000 H2O The reaction mixture obtained in the mixing step is kept at room temperature under stirring for 24 hours.

[0136] The resulting gel is placed in a reactor and heated to 150°C for 6 days with stirring (200 rpm). The crystals are separated and washed with deionized water until the pH of the wash water is below 8. The washed crystallized solid is dried for 12 hours at 100°C. The loss on ignition (LOI) is 18% by weight.

[0137] An XRD analysis shows that the product obtained is a raw, pure, synthetic SSZ-16 structural type AFX zeolite (ICDD datasheet, PDF 04-011-1869).

[0138] Crude SSZ-16 zeolite is calcined under a stream of dry air at 550°C for 12 hours. The calcined SSZ-16 zeolite is then contacted with a 3-molar NH₄NO₃ solution for 5 hours under stirring at room temperature. The ratio of the NH₄NO₃ solution volume to the mass of solid is 10. The resulting solid is filtered and washed, and the exchange procedure is repeated under the same conditions. The final solid is separated, washed, and dried for 12 hours at 100°C.

[0139] The SSZ-16 zeolite in its ammoniacal form (NH4-SSZ-16) is treated under a dry air stream at 550°C for 8 hours with a temperature ramp rate of 1°C / min. The resulting product is the SSZ-16 zeolite in its protonated form (H-SSZ-16). Cu ion exchange on H-SSZ-16

[0140] H-SSZ-16 zeolite is contacted with a [Cu(NH3)4][NO3)2 solution for 12 hours under stirring at room temperature. The final solid is separated, washed, dried, and calcined under a stream of dry air at 550°C for 8 hours. XRD analysis shows that the product obtained is a pure AFX structural-type SSZ-16 zeolite (ICDD datasheet, PDF 04-011-1869).

[0141] X-ray fluorescence (FX) chemical analysis gives a SiO2 / Al2O3 molar ratio of 13 and a Cu mass percentage of 3%.

[0142] The resulting catalyst is designated CuSSZ16. Example 7: NOx conversion to SCR standard: comparison of catalysts according to the invention with the prior art

[0143] A catalytic test of the reduction of nitrogen oxides (NOx) by ammonia (NH3) in the presence of oxygen (O2) under Standard SCR conditions is carried out at different operating temperatures for the catalysts according to example 2 (CuAFX, according to the invention), example 3 (CuAFX780, according to the invention), example 4 (CuAFX720, according to the invention), example 5 (CuAFX600, according to the invention) and the catalyst according to example 6 (CuSSZ16, comparative).

[0144] For each sample test, 200 mg of catalyst in powder form are placed in a quartz reactor. 145 L / h of a load representative of a diesel engine exhaust gas mixture are fed into the reactor.

[0145] This charge has the following molar composition: 400 ppm NO, 400 ppm NH3, 8.5% O2, 9% CO2, 10% H2O, qpc N2.

[0146] An FTIR analyzer allows the concentration of the species NO, NO2, NH3, N2O, CO, CO2, H2O, and O2 to be measured at the reactor outlet. The NOx conversions are calculated as follows: Conversion = NOx entrée - NOx sortie / NOx entrée

[0147] The NOx conversion results under Standard SCR conditions are presented on the Figure 3 The curves CuAFX, CuAFX780, CuAFX720, CuAFX600, and CuSSZ16 correspond respectively to the tests carried out with the catalysts synthesized according to Example 2 (CuAFX, catalyst according to the invention), Example 3 (CuAFX780, catalyst according to the invention), Example 4 (CuAFX720, catalyst according to the invention), Example 5 (CuAFX600, catalyst according to the invention), and Example 6 (CuSSZ16, catalyst not according to the invention). It appears that the catalysts according to the invention allow for the conversion of NOx.

[0148] The CuAFX, CuAFX780, CuAFX720, and CuAFX600 catalysts synthesized according to the invention exhibit superior performance compared to the prior art CuSSZ16 catalyst in terms of NOx conversion across the entire tested temperature range. A maximum conversion rate of 100% is achieved between 259°C and 430°C for the CuAFX780 catalyst, whereas the prior art CuSSZ16 catalyst achieves only 89% conversion between 340°C and 400°C.

[0149] The catalyst ignition temperatures are given below for Standard-SCR conditions: Table 1 T50 T80 T90 T100 CuAFX 180°C 212°C 228°C 310°C CuAFX780 159°C 190°C 204°C 259°C CuAFX720 173°C 202°C 217°C 272°C CuAFX600 172°C 203°C 222°C 283°C CuSSZ16 190°C 257°C 350°C -

[0150] T50 corresponds to the temperature at which 50% of the NOx in the gas mixture is converted by the catalyst. T80 corresponds to the temperature at which 80% of the NOx in the gas mixture is converted by the catalyst. T90 corresponds to the temperature at which 90% of the NOx in the gas mixture is converted by the catalyst. T100 corresponds to the temperature at which 100% of the NOx in the gas mixture is converted by the catalyst.

[0151] The CuAFX, CuAFX780, CuAFX720, and CuAFX600 catalysts synthesized according to the invention exhibit significantly superior performance compared to the prior art CuSSZ16 catalyst in terms of ignition temperatures and NOx conversion across the entire temperature range tested under Standard SCR conditions. Indeed, at the same conversion rate (50% or 80%), the ignition temperatures obtained with the CuAFX catalyst according to the invention are lower than those obtained with the CuSSZ16 catalyst. Example 8: NOx conversion to Fast SCR: comparison of catalysts according to the invention and comparative analysis

[0152] A catalytic test for the reduction of nitrogen oxides (NOx) by ammonia (NH3) in the presence of oxygen (O2) under Fast SCR conditions is performed at different operating temperatures for the catalysts synthesized according to the invention (Examples 2, 3, 4, and 5) and the CuSSZ16 sample synthesized according to the prior art (Example 6). 200 mg of catalyst in powder form is placed in a quartz reactor. 218 L / h of a load representative of a diesel engine exhaust gas mixture is fed into the reactor. This load has the following molar composition: 200 ppm NO, 200 ppm NO2, 400 ppm NH3, 8.5% O2, 9% CO2, 10% H2O, qpc N2 for Fast SCR conditions.

[0153] An FTIR analyzer allows the concentration of NO, NO2, NH3, N2O, CO, CO2, H2O, and O2 to be measured at the reactor outlet. NOx conversions are calculated as follows: Conversion = NOx entrée − NOx sortie / NOx entrée

[0154] The catalyst ignition temperatures are given below for Fast-SCR conditions: Table 2 T50 T80 T90 T100 CuAFX 178°C 210°C 235°C 290°C CuAFX780 162°C 192°C 215°C 270°C CuAFX720 161°C 190°C 214°C 267°C CuAFX600 166°C 200°C 220°C 273°C CuSSZ16 188°C 233°C 269°C 402°C

[0155] T50 corresponds to the temperature at which 50% of the NOx in the gas mixture is converted by the catalyst. T80 corresponds to the temperature at which 80% of the NOx in the gas mixture is converted by the catalyst. T90 corresponds to the temperature at which 90% of the NOx in the gas mixture is converted by the catalyst. T100 corresponds to the temperature at which 100% of the NOx in the gas mixture is converted by the catalyst.

[0156] The CuAFX, CuAFX780, CuAFX720, and CuAFX600 catalysts synthesized according to the invention exhibit superior performance compared to the prior art CuSSZ16 catalyst in terms of ignition temperatures and NOx conversion across the entire temperature range tested under Fast SCR conditions. Indeed, at the same conversion rate (50%, 80%, 90%, or 100%), the ignition temperatures obtained with the CuAFX catalyst according to the invention are lower compared to those obtained with the Cu-SSZ-16 catalyst.

[0157] Furthermore, the emissions of nitrous oxide (N2O), in the case of the CuAFX catalyst according to the invention, remain low over the entire temperature range tested (<20ppm between 150 and 550°C).

Claims

1. Process for preparing a catalyst based on a zeolite of AFX structural type and on at least one transition metal, comprising at least the following steps: i) the mixing, in an aqueous medium, of at least one source of at least one silicon oxide SiO2, of at least one source of at least one aluminium oxide Al2O3, of an organic nitrogen-comprising compound R, also referred to as specific structuring agent, 1,6-bis(methylpiperidinium)hexane dihydroxide, of at least one alkali metal and / or one alkaline earth metal M with a valency n, n being an integer greater than or equal to 1, the reaction mixture having the following molar composition: SiO2 / Al2O3 between 2 and 100, preferably between 12 and 40 H2O / SiO2 between 5 and 60, preferably between 10 and 40 R / SiO2 between 0.05 and 0.50, preferably between 0.10 and 0.40 M2 / nO / SiO2 between 0.05 and 0.40, preferably between 0.15 and 0.30, wherein M is sodium, step i) being conducted for a duration that allows a homogeneous mixture, referred to as precursor gel, to be obtained; ii) the hydrothermal treatment of said precursor gel obtained at the end of step i) under autogenous pressure at a temperature of between 120°C and 250°C, preferably between 150°C and 230°C, for a duration of between 2 and 12 hours, preferably between 2 and 10 hours, until said zeolite of AFX structural type forms, iii) at least one ion exchange, comprising bringing said solid obtained at the end of the preceding step into contact with a solution comprising at least one species capable of releasing a transition metal, in solution in reactive form with stirring at ambient temperature for a duration of between 1 hour and 2 days, the transition metal being copper and / or iron and the content of transition metal(s) introduced by the ion exchange step iii) being between 0.5% and 6% by mass, preferably between 0.5% and 5% by mass, more preferably between 1% and 4% by mass, relative to the total mass of the anhydrous final catalyst; iv) a heat treatment advantageously comprising drying the solid obtained at the end of the preceding step (ii) or (iii) at a temperature of between 20 and 150°C, preferably between 60 and 100°C, for a duration of between 2 and 24 hours, followed by at least one calcination under - optionally dry - air at a temperature of between 450 and 700°C, preferably between 500 and 600°C, for a duration of between 2 and 20 hours, preferably between 6 and 16 hours, more preferably between 8 and 13 hours, the flow rate of optionally dry air being preferably between 0.5 and 1.5 L / h / g of solid to be treated, more preferably between 0.7 and 1.2 L / h / g of solid to be treated.

2. Process for preparing a catalyst according to Claim 1, wherein at least one source of at least one silicon oxide SiO2 and / or at least one source of at least one aluminium oxide Al2O3 is at least one zeolite of FAU structural type having a SiO2 / Al2O3 molar ratio of between 2.00 and 100.

3. Process according to either of Claims 1 and 2, wherein steps iii) and iv) are reversed and / or optionally repeated.

4. Process for preparing a catalyst according to one of Claims 1 to 3, wherein seed crystals of a zeolite of AFX structural type are added to the reaction mixture of step i), preferably in an amount of between 0.01% and 10% of the total mass of the sources of said tetravalent (Si) and trivalent (Al) elements in their oxide form (SiO2 and Al2O3) which are used in the reaction mixture, said seed crystals not being taken into account in the total mass of the sources of the tetravalent and trivalent elements.

5. Process for preparing a catalyst according to one of the preceding claims, wherein step i) comprises a step of maturation of the reaction mixture at a temperature of between 20 and 80°C, with or without stirring, for a duration of between 30 minutes and 24 hours.

6. Process for preparing a catalyst according to one of the preceding claims, wherein step iii) of ion exchange is carried out by bringing the solid into contact with a solution comprising a single species capable of releasing a transition metal or by successively bringing the solid into contact with different solutions each comprising at least one, preferably a single, species capable of releasing a transition metal, the transition metals of the different solutions preferably being different from each other.

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